A modern integrated circuit (IC) must meet very stringent design and performance specifications. In many applications for communication devices, transmit and receive signals are exchanged over communication channels. These communication channels include impairments that affect the quality of the signal that traverses them. One type of IC that uses both a transmit element and a receive element is referred to as a serializer/deserializer (SERDES). The transmit element on a SERDES typically sends information to a receiver on a different SERDES over a communication channel. The communication channel is typically located on a different structure from where the SERDES is located. To correct for impairments introduced by the communication channel, a transmitter and/or a receiver on a SERDES or other IC may include circuitry that performs channel equalization. Channel equalization is a broad term that comprises many different technologies for improving the accuracy of communication between a transmitter and a receiver. One typical type of equalization is referred to as decision feedback equalization and is performed by a decision feedback equalizer (DFE). A DFE is typically implemented in a receiver and improves the signal-to-noise ratio (SNR) of the signal, but it can suffer from burst error propagation.
A feed forward equalizer (FFE) does not suffer from burst error propagation, but nor does it provide the improvement in SNR as does a DFE.
Additionally, a DFE can only be utilized for post cursor equalization, where a FFE can be used for either or both of pre or post cursor equalization.
Moreover, these drawbacks become more pronounced when attempting to design and fabricate a receiver that can operate using both PAM 2 and PAM 4 modalities. The acronym PAM refers to pulse amplitude modulation, which is a form of signal modulation where the message information is encoded into the amplitude of a series of signal pulses. PAM is an analog pulse modulation scheme in which the amplitude of a train of carrier pulses is varied according to the sample value of the message signal. A PAM 2 communication modality refers to a modulator that takes one bit at a time and maps the signal amplitude to one of two possible levels (two symbols), for example −1 volt and 1 volt. A PAM 4 communication modality refers to a modulator that takes two bits at a time and maps the signal amplitude to one of four possible levels (four symbols), for example −3 volts, −1 volt, 1 volt, and 3 volts. For a given baud rate, PAM 4 modulation can transmit up to twice the number of bits as PAM 2 modulation.